The present invention relates to monopulse radar and more particularly to the monopulse radar traditionally used for airborne weather detection.
Anticipated increases in air traffic volume, as well as economic pressure to reduce airline operating costs are spurring the development of an air transportation system that operates at maximum capacity under all visibility conditions. Several hazards to the safety of flight, however, present themselves or are exacerbated during reduced visibility operations. These hazards include: impact with the terrain surrounding the airport; failure to acquire the intended runway for landing; and failure to detect obstructions that may be on the runway, taxiway or otherwise in the path of the aircraft. For these reasons, the air traffic control system imposes minimum cloud ceiling and runway visibility requirements at airports that mandate under what conditions the airport can accept landing and departing traffic. These minima are designed to ensure that the flight crew has enough information to acquire the correct runway and avoid collision hazards on departure and approach.
In addition, low visibility operations at airports also require that air traffic control separate landing and departing traffic from each other by a greater distance. The net effect of the increased separations is to reduce the number of aircraft the airport can handle in a given time period.
Relaxation of the given minima at an airport is possible if both the aircraft and the airport have sophisticated precision guidance equipment. The precision guidance equipment (e.g. instrument landing system (ILS) or microwave landing systems) improves the confidence with which the aircraft can acquire and maintain the proper flight path to the correct runway. Airports having this precision guidance equipment can enjoy improved capacity during times of low visibility over airports without this equipment. However, this equipment is expensive to acquire and maintain and many airports do not have equipment of this type. Furthermore, these systems require specialized equipment both on board the aircraft and at the airport. In addition, use of these systems still do not provide the airport with the same capacity present during times of unrestricted visibility as hazards to flight due to the reduced visibilities still exist.
Certain dedicated systems are currently manufactured to warn of these potential hazards. Chief among these systems are those designed to prevent controlled flight into terrain accidents. Controlled flight into terrain accidents currently account for the greatest number of air fatalities, the risk of which is greatly increased by operations in low visibility conditions. Technology for avoiding controlled flight into terrain includes ground proximity warning systems, and terrain awareness and display systems.
Ground proximity warning systems use altitude information from radio altimeters and barometric altimeters, in conjunction with an individual aircraft's speed and climb characteristics, to warn flight crews that the terrain below the aircraft is rising dangerously fast. The ground proximity warning systems can also provide an aircraft flight crew with additional alerts by, for example, warning of aircraft deviation below glideslope or inappropriate aircraft attitude or configuration. Typical examples of ground proximity warning systems are disclosed in U.S. Pat. No. 3,946,358 entitled "Aircraft Ground Proximity Warning Instrument" and U.S. Pat. No. 4,914,436 entitled "Ground Proximity Approach Warning System Without Landing Flap Input," both incorporated herein by reference.
Terrain awareness and display systems combine ground proximity warning system technology with navigation data, a built-in terrain data base and existing cockpit display technology such as color weather radar, electronic flight instrument systems (EFIS) and map displays. Terrain awareness and display systems provide "look ahead" terrain warnings by utilizing present aircraft positions and a terrain data base to predict the aircraft's future position with respect to terrain. A typical example of a terrain awareness system is described in co-pending application Ser. No. 08/509,642, filed Jul. 31, 1995, entitled "Terrain Awareness System" by Muller et al, assigned to the same assignee as the present application.
Although the ground proximity warning systems and terrain awareness and display systems described in the above-mentioned references have greatly reduced the controlled flight into terrain risk for aviation worldwide, both ground proximity warning systems and terrain awareness and display systems have some limitations. Neither of these systems actually "sees" the terrain or other obstructions ahead of the aircraft. Ground proximity warning systems differentiate the aircraft's altitude signals to detect abnormally high closure rates with terrain. Thus, discontinuities in the terrain profiles, such as a cliff, may not generate an alert in sufficient time to prevent an accident. The more sophisticated "look ahead" function of terrain awareness and display systems compares aircraft position data, based on either dead reckoning or a global positioning system, with a stored terrain map to calculate the aircraft's probable position relative to the terrain and determine whether a terrain collision threat exists. However, this system cannot detect collision threats due to obstructions not contained within the database. For example, temporary structures such as construction cranes would not be modeled in the database. In addition, the integrity of the alerting function depends directly upon the integrity of the aircraft position data. Errors in aircraft position could reduce the warning time given the flight crew. In addition, non-fixed terrain features and non-fixed terrain threats such as, for example, aircraft or vehicular traffic on the runway, are also not readily determinable by typical ground proximity warning systems. Thus, these systems are inappropriate as a means for relaxing airport visibility minima and increasing airport capacities.
Radar has the potential to provide the flight crew with real-time terrain information independent of both a calculated position and a computer-stored terrain data base. However, the only radar normally carried aboard non-military aircraft is weather radar. Weather radar has characteristics that make it non optimal for detecting terrain threats specifically. Existing weather radar antennas exhibit a limited elevation sweep angle. The added weight and expense of a radar dedicated to terrain detection in addition to the already required weather radar prohibits use of terrain only radar systems. Yet, additional safety and increases in airport capacity could be realized through the use of this additional radar information.
Utilizing an aircraft radar for detection of these threats, poses unique difficulties. Effective real-time terrain feature identification and terrain threat determination require resolution of closely spaced targets, for example, closely spaced radio towers. The typical monopulse radar antenna transmits through the Sum channel and receives data through Sum and Delta channels. Nearly all current radar applications utilize only these two Sum and Delta channels, which are manipulated to obtain target off-boresight angle information. Radar applications utilizing the Sum and Delta channels exclusively are incapable of resolving closely spaced targets. If multiple targets are present in the beam or if the target is widely distributed, the monopulse measurement becomes confused. Traditional monopulse angle measurement is unable to separate two closely spaced targets; known monopulse sharpening techniques may even degrade the image. Thus, it has been difficult to distinguish multiple targets or widely distributed targets when they are concurrently present in the radar beam. Resolving closely spaced targets is valuable in achieving radar-based autonomous landing guidance.